1. Field of the Invention
The present invention relates to a mounting for an elongate rotary body, such as a turbine shaft of a turbo machine.
2. Description of the Prior Art
Rotary shafts of compressors or turbines for example, may be mounted by two radial bearings of hydraulic, pneumatic or roller type, disposed in the vicinity of the ends of the shaft. In particular, sealed hydraulic type bearings filled with oil or a viscous fluid have been proposed for this purpose.
In order to increase the performance of a compressor, turbine or other machine it is desirable to increase the length of the shaft. However, it is not possible to mount a shaft of substantial length only by means of two end bearings, as the deformations of the shaft then become too great and the critical rotation frequencies too low. Consequently, it is found necessary to provide for an additional support of the shaft at an intermediate position between the two end bearings. However, the diameter of the shaft must remain substantially greater in the central portion, as otherwise the rigidity of the shaft might become too low. This rules out the utilisation of an additional hydraulic bearing. Indeed, a hydraulic bearing mounted around a shaft of large diameter gives rise to too great a braking action. In addition, the installation of a hydraulic bearing in the central portion of the shaft would involve the presence of seal packings in the middle of the machine.
These difficulties in mounting an adequate support at the centre of the machine have given rise to shafts of restricted length, and, to increase the performance of a machine, it is necessary to couple two shafts in series. Each shaft, mounted in an independent body, is supported by two end bearings. Seal packings are disposed at both ends of the shaft.
It has been proposed to mount vertical shafts of substantial length, for example vacuum pump rotor shafts, with the aid of an assembly of magnetic bearings. Such a type of assembly is described for example in French Pat. No. 2,052,314. Such a mounting comprises at one or each of the ends of the shaft, a supporting magnet which generates a constant magnetic field. In addition, auxiliary devices for the stabilisation of the shaft in a radial direction, which may possibly be placed away from the supporting magnets or electro-magnets, cooperate with the latter to ensure a stable holding of the shaft. However, since the shaft has a substantially vertical axis of rotation, the supporting magnet or magnets generate a constant magnetic field with a vertical component which holds the shaft in a stable position vertically, while the stabilisation devices, which are not supporting, ensure only radial stabilisation in one or more horizontal planes, and use reduced power. The stabilisation devices which comprise at least one detector of the radial position of the shaft, electro-magnetic means for generating magnetic fields acting on radially orientated ferromagnetic parts of the shaft, as well as an apparatus for controlling the electromagnetic means from the signals supplied by the detectors, are provided essentially, not to generate a magnetic field proper of a certain value, but to modify the magnetic field of the supporting magnets. In certain cases, when the vertical shaft is very long, and not very rigid, one or two horizontal stabilisation units may be distributed along the rotor to damp oscillations. In the latter case, the shaft itself must be ferromagnetic, and the stabilisation units cooperate directly with the shaft.
Thus, with this previously proposed mounting, it is possible to utilise shafts of great length only in a vertical orientation, with an end bearing, of passive type, which effects the support of the shaft, while one or more low power stabilisation magnetic devices, of active type, contribute to a horizontal stabilisation of the shaft. In addition, as a consequence of the separation of the support and stabilisation functions, this previously proposed mounting is suitable for shafts having low bending rigidity only.